Both have crushed the market this year.Long-term investing is easy and relatively stress-free, at least in principle. All you have to do is purchase shares of quality companies and sit back and watch for years -- sometimes decades -- as the value of your investment rises. However, there are several factors that make this strategy somewhat more complicated than it appears.One of them is the challenge of picking which stocks to invest in. Making the wrong choice will not lead to a stress-free ride -- quite th ...